The present invention relates to a process for roughening a substrate for photosensitive layers, the surface of which substrate is roughened mechanically and subsequently electrochemically in an aqueous electrolytic bath by applying a three-phase or alternating current to the electrodes opposite the substrate.
Such substrates are employed for the production of presensitized printing plates, the material of the substrates, which are processed into the form of plates or webs, being a metal, especially aluminum. Roughening of, for example, aluminum webs for the production of printing plates is done mechanically, electrochemically or in a combination of a mechanical and electrochemical roughening process. In this regard, the aim is for the aluminum surface to have a specific structure and uniformness, for it must readily accept water and at the same time ensure good adhesion of the photosensitive layer. In the case of mechanical roughening, the surface structures are pyramid-like forms, while electrochemically roughened aluminum surfaces have a sponge-like structure with many cells and depressions.
By comparison with purely electrochemical roughening, mechanical roughening has the advantage of lower specific energy consumption per square meter of substrate surface, but the disadvantage of producing too coarse a surface on which crystalline structures are still present in addition to the pyramidal structures.
Mechanical roughening processes are, in general, processes, such as wire or brush graining, or emery grinding, whereas electrochemical roughening is done, in general, through electrolytic etching in an aqueous electrolytic solution.
German Patent 1,962,728 describes a process for the continuous manufacture of a lithographic surface on a metal web by wet grinding and electrochemical treatment in an electrolyte, in which process the electrolyte is employed to wet the metal surface during grinding and the electrochemical treatment is carried out directly after the grinding. For this purpose, a fine-grained abrasive is suspended in the electrolyte, and the abrasive suspension is blasted onto the moving web in a wide jet extending over the entire width of the metal web. The electrolyte is, for example, an aqueous acidic or aqueous alkaline bath.
In the case of the graining process described in German Offenlegungsschrift 2,130,391, the aluminum plate is first roughened by grinding with a moist emery composition, and after rinsing and, if necessary, cleaning of the plate, the grained surface of the aluminum plate is anodized in a sulfuric acid solution with direct current at a voltage in the region of approximately 10 to 20 V and a current density in the region of approximately 1 to 2.2 A/dm.sup.2. Finally, the grained and anodized surface of the aluminum plate is treated with an primer substance for improving the bonding of the photosensitive layer to be applied to the surface to the substrate.
German Auslegeschrift 2,650,762 discloses a process for electrolytic graining of aluminum substrates for lithography by means of an alternating current in an electrolyte essentially containing hydrochloric acid or nitric acid, the alternating voltage applied in this process being such that its anode voltage is greater than the cathode voltage and the ratio of the cathodic coulomb input to the anodic coulomb input is less than 1. The anode alternation of the alternating current is set to be equal to or less than the cathode alternation. The diameter and the depth of the pores or pits in the surface of the aluminum substrate can be predetermined by selecting a suitable ratio of the cathodic to the anodic coulomb input as determined by the voltage setting. The frequency of the regulated alternating current is not limited to the usual frequency range of alternating current, i.e., 50 to 60 Hz. Finer pores are obtained on the grained surface with higher frequencies.
German Patent Specification 3,012,135 describes a process for producing a substrate for lithographic printing plates, in which process the surface of an aluminum plate is mechanically roughened by wet grinding, aluminum is chemically etched from the surface of the plate, and subsequently an electric current having a waveform which is obtained by alternating change in polarity, is applied to the plate in an acidic aqueous solution in such a way that the ratio of the amount of charge formed with the plate as anode to the amount of charge formed with the plate as cathode is 0.5:1 to 1 0 1. The electrolysis is carried out in such a way that, if the plate is the anode, the current density amounts to not less than 20 A/dm.sup.2, and the amount of charge formed with the plate as anode amounts to 200 coulomb/dm.sup.2 or less, and the anode and cathode voltages are 1 to 50 V. Finally, the plate is subjected to an anodic surface oxidation.
In combining the mechanical and electrochemical roughening the aim is to bring together the advantages of the two processes.
It is expected that the mechanically roughened surface of the metal substrate is finely superposed by cells and depressions, which result from the electrochemical roughening. However, in this regard it emerges undesirably that apart from the pyramidal structures of the mechanical roughening, relatively large pits occur, which are the result of the electrochemical roughening. In order to attain results which are halfway useful, it is necessary for the mechanical roughening to be followed by a disproportionately intense electrochemical roughening, leading to a very steep rise in current consumption which is caused by the resulting pits of the electrochemical roughening. The cause of the pits is too intense and too long an effect of the current which, on the other hand, is required, in turn, in order to arrange the distribution of the pits very uniformly. Just as problematical in the case of the superposition of the mechanically roughened surface of a metal substrate with electrochemical roughening by means of alternating current at a very high working rate of the metal substrate is the formation of so-called electrical cross-strokes in step with the alternating current voltage, these cross-strokes being visible in the form of strokes on the surface of the metal substrate. The cause of these disturbing cross strokes is in all likelihood the continual change in polarity of the alternating current applied at the electrodes.